This section illustrates useful background information without admission of any technique described herein being representative of the state of the art.
Sensitivity of optical detection, for example in laser spectroscopy, can be improved by increasing the optical pathlength within the sample being analyzed. Such an increase of optical pathlength is conveniently obtained using a multipass cell. Conventional multipass cells, e.g. Herriott cells, are well known. Such conventional multipass cells require a large volume and are accordingly unsuitable for applications with limited space and/or low available sample volume.
Previously a multipass cell with circular beam pattern has been disclosed in patent publication U.S. Pat. No. 7,876,443 B. Such a multipass cell can be fitted into a relatively low volume. However, such a cell is difficult to produce even with modest optical surface quality and accordingly such a cell is expensive.
A circular multipass cell constructed from separate sections has also been previously presented in a journal article “Versatile multipass cell for laser spectroscopic trace gas analysis” by Manninen et. al. in Applied Physics B (2012) 109, p. 461-466. However, such a cell, while more cost-effective, requires very tasking alignment of the separate mirrors and problems arise with diffraction on the mirror edges.
Furthermore, concave mirrors opposite to one another have been disclosed in patent publication U.S. Pat. No. 5,786,893 for use in focusing and pumping laser light from several sources into a sample region of a Raman spectrometer.
It is the aim of the current invention to provide a multipass cell that mitigates for example the above problems of the state of the art and/or provides a high optical path to sample volume ratio.